Systematic Extraction Flow and Gate Stack Development for GaN HEMT Devices with MVSG Model

Abstract

Gallium Nitride (GaN) High Electron Mobility Transistors (HEMTs) are rapidly being featured as core components in modern high voltage and high frequency systems due to their wide band gap, high breakdown voltage, and superior electron mobility. New topologies and technologies are rapidly emerging to address industrial challenges. The MIT Virtual Source GaN (MVSG) model is a physics based compact model that accurately predicts device behaviour for a wide variety of effects seen in GaN HEMTs.

A central contribution of this work is the development of a systematic extraction flow that is easy to understand, yet thoroughly calibrates a model to the highest potential. It features extraction of contact resistances using transmission line model (TLM) structures, capacitances with various small signal analyses, and a thorough channel transport extraction, all with temperature coefficients and guidelines on measurement techniques. Calibration of the thermal module is considered throughout the extraction flow, allowing better isolation between different parameters. A robust way to obtain the thermal capacitance is also present, further enhancing the utility of the model. For the first time ever, an extraction flow has been added to a charge trapping model, requiring only transient measurements to be obtained, but allowing all parameters to be obtained accurately. Finally, small signal S-parameters, noise, and large signal power response is included as well.

To support further development of the GaN HEMT industry, the MVSG model has been augmented with a more robust high injection model and p-GaN module -- both of these a first in compact modeling. The high injection model is robust and computationally simple, producing textbook quality curves. The p-GaN module, based in physical principles, allows modeling of Schottky gated devices, as well as hybrid devices, offering a wide range of applicable use cases. It accurately predicts behaviours seen all across industry, including the voltage division, charge storage, and transient $V_{\textrm{t0}}$ shift effects. With the rapid commercialization of enhancement mode devices, this model is essential to circuit design simulations such as power converter design.

The MVSG model has been adapted and used extensively to characterize GaAs devices, using the same extraction methodology for GaN devices. The predictive capabilities of the model were put to the test with the availability of different device dimensions, where excellent agreement with measurements was achieved.

Future work from this thesis includes further refinement of the p-GaN module and parameter extraction. Geometry dependent thermal module parameters are also an area of interest. Bidirectional GaN HEMTs, with symmetrical IV capabilities, are also starting to be developed, using the p-GaN technology that was developed in this thesis.